Mobile phones, laptop personal computers and like systems that are made portable by supplying power by means of a battery have been spread in use. Such systems have adopted various power-save technologies for suppressing power to be consumed in order to effectively use the limited power. Such power-save technologies include those for suppressing the power to be consumed for each of devices contained in the system and those for suppressing the power to be consumed in view of the entire operation of the system (see Japanese Unexamined Patent Publication No. H08-314587 (page 7, FIG. 1).
However, the conventional power-save technologies have a problem that a fluctuation in the consumed power of the entire system per unit time cannot be suppressed when the operative state of the power-consuming device changes and accordingly, the consumed power of this device fluctuates. Batteries have such a characteristic that a charged energy is lost within a shorter time as the consumed power largely fluctuates even if a total amount of the consumed power is same. Accordingly, the conventional power-save technologies cannot suppress the waste of the charged energy of the battery resulting from the fluctuation in the consumed power, thereby presenting a problem of unnecessarily shortening the life of the battery (i.e., period until the charged energy is lost).